Formula
kVA is apparent power. kW is real power. Power factor is the ratio. Single-phase current is kVA times 1,000, divided by the voltage. Three-phase current divides by √3 as well, because the same kVA is carried on three lines. √3 is about 1.732. The turns ratio is primary voltage divided by secondary voltage.
I = kVA × 1,000 / V
I = kVA × 1,000 / (√3 × V)
kVA = kW / PF × (1 + margin ÷ 100)
A 480 volt to 208Y/120 volt transformer is a common three-phase step-down. A 120/240 volt secondary is the usual single-phase split. The nameplate kVA is the rating. Impedance percent is also on the nameplate and is not used here. Continuous load and future load are why a margin is offered. Twenty to twenty-five percent is a planning allowance you can type. It is not a requirement of Article 450.
Worked examples
75 kVA, 480 V primary, three-phase. 90.21 A. The 208 V secondary of that same transformer is 208.18 A. 45 kVA at 208 V, three-phase. 124.91 A.
25 kVA, single-phase. 104.17 A at 240 V, and 52.08 A at 480 V.
240 V × 80 A, single-phase. 19.2 kVA, next standard size 25 kVA. 60 kW, three-phase, power factor 0.9. 66.67 kVA, standard size 75. With a 25 percent margin, 83.33 kVA rounds to 112.5.
Turns ratio. 480 : 240 is 2 : 1. 480 : 208 is 2.308 : 1.
Reference
Standard sizes this page will round up to. A calculated kVA that lands on a rung stays there.
| Single-phase kVA | Three-phase kVA |
|---|---|
| 3 | 3 |
| 5 | 6 |
| 7.5 | 9 |
| 10 | 15 |
| 15 | 30 |
| 25 | 45 |
| 37.5 | 75 |
| 50 | 112.5 |
| 75 | 150 |
| 100 | 225 |
| 167 | 300 |
| 250 | 500 |
| 333 | 750 |
| 500 | 1000 |
Questions
How do you calculate transformer kVA?
Single-phase kVA is volts times amps, divided by 1,000. Three-phase kVA multiplies by √3 first, because the line current is shared across three phases. The other direction divides kVA by the voltage, with the same √3 when the transformer is three-phase.
How many amps is a 75 kVA transformer?
At 480 volts, three-phase, full-load current is 90.21 amps. At 208 volts on the same kVA it is 208.18 amps. Single-phase 75 kVA is a different number, because the √3 is not in that formula.
How do I size a transformer?
Divide the real load in kilowatts by the power factor, then add any growth margin you choose. Round up to the next standard kVA. Sixty kilowatts at 0.9 power factor is 66.67 kVA, which lands on a 75 kVA three-phase unit. A 25 percent planning margin makes 83.33 kVA and the next standard size is 112.5.
What is the difference between kVA and kW?
Kilowatts are real power, the part that does work. Kilovolt-amperes are apparent power, voltage times current. Power factor is kW divided by kVA. A motor nameplate is often near 0.9. A heater is close to 1.0.
What size transformer do I need for 100 amps?
It depends on the voltage and the phase. One hundred amps at 240 volts, single-phase, is 24 kVA, so the next standard size on this page is 25 kVA. The same 100 amps at 208 volts, three-phase, is about 36 kVA, which rounds to 45 kVA. Enter the voltage you actually have.
What are standard transformer sizes?
This page rounds up through a ladder taken from IEEE C57.12.00 Table 3, plus the small dry-type steps 3, 5, and 7.5 kVA single-phase and 3, 6, and 9 kVA three-phase. A calculated 19.2 kVA becomes 25 kVA. A size between two rungs is the higher rung.
How do I convert three-phase kVA to amps?
Multiply kVA by 1,000 and divide by √3 times the line-to-line voltage. That is the full-load line current.
Sources
- IEEE C57.12.00, Table 3, preferred continuous kVA ratings. The single-phase ladder from 5 kVA upward on this page, and the three-phase ladder from 15 kVA upward, follow that table. The small dry-type steps 3, 5, and 7.5 kVA single-phase, and 3, 6, and 9 kVA three-phase, sit below the first rows of that table and are included so a small load still has a rung.
- Eaton low-voltage dry-type distribution catalog. Single-phase ventilated units are listed from 15 kVA, and three-phase units include 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, and 1,000 kVA. Those are the same rungs as the upper part of this ladder.
- NEC 2023, Article 450, and Table 450.3(B) for transformers of 1,000 volts and less. Cited for context only. This page does not select overcurrent protection or a conductor.
- NIST SP 811 for the volt, ampere, and watt. √3 is the exact square root of 3.